In reciprocating hermetic compressors with small displaced volume, the discharge valve directly affects compressor energy and volumetric losses.
The discharge valve energy loss is related to a process for opening the valve and to the gas discharge period. In the opening process, an overpressure loss is basically caused by the valve inertia, the adhesion between the valve blade and the valve plate due to oil presence and blade pre-tension. During the gas discharge period, losses are related to charge loss due to flow restrictions and to the power spent to maintain the valve blade spaced from the seat since said blade is prone to close because it has a spring effect.
Both the volumetric loss and the energy loss are affected as well by the gas retained in the discharge passage and the gas coming back from inside the cylinder due to the later valve closing.
As hereinabove described, it could be said that an accurate definition of constructive characteristics of the outlet valve is an important aspect for sizing the hermetic compressor. Usually valves which are available for this type of compressor are for example those described in the following documents U.S. Pat. No. 4,723,896 and Brazil PI 8400733 (DE P 3305791.5/83).
In either of the prior arrangements, a recess is used in the valve plate which extends along the region of the discharge orifice and into which the blade itself an additional reinforcement blade are inserted, the latter blade being overlapped on the first blade in order to give a spring effect upon the assembly. The valve is completed by using a backstop limiting the displacement and helping the attachment of the blade and the additional blade and/or reinforcement.
In the prior valve, the constructive form is such that from, a predetermined displacement step of the blade, there will be contact with the reinforcement blade, and the assembly will have its inertia greatly increased, this to be overcome and supported by the gas to be discharged. This characteristic additionally increases the energy needed in relation to that which is necessary to simply expel the gas from inside the cylinder. Therefore, this will reduce the energy efficiency of the compressor.
In reciprocating compressors using air as an operating fluid, mainly those with a large displaced volume, discharge valves formed of a spring overlapped upon a disc acting as a seal when seated on the orifice are used. In this case, the spring is made so as to have only one stiffness along the whole course of the disc opening.
Another type of discharge used in hermetic compressors for small refrigerating machines is described in the Brazilian Patent Application No. 8900196. In this case, the discharge valve is formed of a spring, one end of which overlappingly engaged a disc which will act as a seal when seated in the orifice and a backstop mounted on the cylinder cover and the inside of which the other spring end engaged. This backstop has also the purpose of guiding the spring and defining the maximum axial displacement separating the sealing disc from the orifice.
In this type of assembly a great number of tolerances are involved. Thus, in a given assembly said spring could be exerting a pre-tensioning on the disc thereby making it difficult to open a valve and a power loss is caused by overpressure.
In another assembly the spring could be so far from the disc so as to allow a great quantity of gas to return to the cylinder, since the disc will seat in the orifice only when a sufficient pressure differential occurs between the discharge chamber and the cylinder, thereby causing a volumetric loss.